In the fabrication of doors and windows (hereafter called units) back bedding sealants are used to seal and bond panes of glass to retain the pane in position within the frame, to provide a weather proof seal and, in some cases to reinforce the structural strength of the unit. In a typical application, the back bedding is applied to the glazing legs of the unit, the glass pane is "dropped" onto the sealant-bearing glazing legs and stops are installed against the glass face opposite the face in contact with the sealant. The sealant fills the gap between the glazing legs and the glass thus providing a seal against water and air infiltration. If the sealant has sufficient adhesive and bonding properties, it will also bond the glass to the unit. This bonding function will contribute significantly to the overall structural strength of the unit.
Structural strength is necessary in all windows and doors in order to keep them intact, leak free and to prevent permanent deformation when subjected to forces caused by wind pressure and opening and closing. When sash and frame components are of sufficient strength to resist such forces, the glass needs only to be sealed in the unit to prevent leakage. In some applications, however, windows and doors can be made with sash and frame components which are not of sufficient strength unless bonded to the glass. In these applications, bonding the glass to the frame components reinforces the frame components so that the overall structural strength of the unit is adequate for the particular application. The use of back bedding sealant compounds which will bond to glass has become prevalent because of lower component costs due to reduced material requirements.
Back bedding sealants which serve only to seal the glass are commonly referred to as ductile back bedding sealants. Those which also bond the glass and thereby affect the overall strength of the unit are referred to as bonding type back bedding sealants. The present invention relates to bonding type back bedding sealants.
Water based sealants have been used for back bedding for many years but their use was limited to less severe applications that did not require bonding of the glass for strength. Over the last several years, water based bonding type back bedding sealants have been introduced which add some strength to the unit. These sealants have generally, however, not had the combination of rheological, tensile, adhesive and low temperature properties required for many applications. For example, known pigmented water based sealants may provide low temperature flexibility and high temperature tensile strength. However, these sealants do not provide high levels of adhesion and thus do not fail cohesively in a peel test unless heated to 100.degree. F. or higher for at least 24 hours prior to measuring the adhesion. If left to cure at temperatures below 100.degree. F., even for several weeks, these sealants will not attain the desired strength or cohesive failure properties. This is a significant disadvantage since windows and doors frequently are not exposed to elevated temperature for many weeks or months after being placed in service. In the alternative, the unit can be placed in a curing chamber at elevated temperature to facilitate the development of adequate bond strength. This step entails extra time, expense and bother, making a bedding sealant that will develop adequate bond strength at low (less than 85.degree. F.) temperature highly desirable.
The problem of obtaining the desired strength and cohesive failure properties at ambient curing temperatures has been a serious drawback to using water based back bedding sealants. Polymers and combinations of polymers have been used in the field of heat-activated adhesives but these formulations must be cured at temperatures ranging from 175.degree.-275.degree. F. before the desired bond strength and adhesion are developed. This makes the formulations unsuitable for use as back bedding sealants, as the units are typically allowed to cure at ambient temperature.
Other known clear water based sealants may have low temperature flexibility and cohesive failure in the peel test without heating. These sealants, while adequate, do not have the adhesive strength and tensile strength of the present invention, particularly at elevated temperatures.
Other water based sealants have rheologies which are suitable for joint sealing applications but not for back bedding. The use of these sealants in back bedding applications requires special care during the assembly process to avoid excessive squeeze out of the sealant when it is compressed between the glass and the frame.
Thus, there exists a need for an improved bonding type back bedding sealant with improved rheology and tensile properties which develops adequate adhesion and bond strength when the unit is cured at low (less than 85.degree. F.) temperature.